1. ATMOSPHERE Air Circulation
UNIT 7
Meteorology and Climate

2. Atmosphere-Ocean Coupling
Why study atmospheric circulation?
Atmosphere and ocean processes are intertwined
Atmosphere-ocean interaction moderates surface temperatures, weather and climate
Weather: local atmospheric conditions
Climate: regional long-term weather
Atmosphere drives most ocean surface waves and currents

3. Composition of the Atmosphere
Dry Air: 78% Nitrogen, 21% Oxygen
BUT it is never completely dry
Typically contains about 1% water vapor
Chemical residence time of water vapor in the air is about 10 days
Warm air holds much more water vapor than cold air

4. Density of Air Typical air density ~ 1 mg/cm3
Temperature and pressure affect the density of air
Temperature: Hot air is less dense than cold air
Pressure: Air expands with elevation above sea level

5. Density and Temperature
Rising air expands and cools
Vapor condenses into clouds and precipitation
Sinking air is compressed and warms
Clear air

6. Expanding Air Cools and Condenses
Like opening a pressurized bottle of soda
Air expands and cools
Water vapor condenses – cloud formation

7. Solar Heating of the Earth
Solar energy absorbed unevenly over Earth’s surface – why is air rising?
Energy absorbed / unit surface area varies with:
Angle of the Sun
Reflectivity of the surface (i.e., ice versus ocean)
Transparency of the atmosphere (i.e. clouds)

8. Solar Insolation Variations with Latitude

10. Solar Heating of the Earth
Sunlight heats the ground more intensely in the tropics than near poles
July
January

11. Solar Heating and Seasons
Seasons are caused by Earth’s 23.5° tilt

12. Solar Heat Energy
Equator absorbs more heat from the sun than it radiates away
Polar regions radiates much more heat than they absorb from the sun
Energy in at equator and heat out at poles
Heat transfer from
E.g. Equator isn’t that hot – Poles aren’t that cold
Evidence that the atmosphere (2/3) and oceans (1/3) redistribute heat (wind and ocean currents)
Convective heat transfer moderates Earth climate

13. Convective Heat Equator Poles
Convective heat transfer model’s Earth climate
Heated air expands and rises, then cools and sinks
Equator
Poles

14. Atmospheric Circulation
Cold, more dense air sinks near the Poles
Warm, less dense air rises near the Equator
Cold, more dense air sinks near the Poles
Wind from the north

15. Actual Atmospheric Circulation
Air rises and sinks
More than one convection cell
Earth spins once per day that amounts to a speed for us at the surface of Earth of 100’s of miles/ hour
Coriolis Effect

16. Coriolis Effect Movies

17. The Coriolis Effect on Earth
Surface velocity increases from poles to equator
Points on the equator must move faster than points near the poles to go around once a day
Latitude velocity differences land to curving paths
Northern hemisphere deflected to right

18. The Coriolis Effect Strength of Deflection varies with latitude:
Maximum at the poles
Zero(!) at equator
Faster a planet rotates, the stronger the Coriolis effects
The larger the planet, the stronger the Coriolis effects (Jupiter spins once every 10 hours)

19. Hurricanes
A storm with lots of clouds has rising air – thus low- pressure at the surface!
Converging air sets up counter-rotation (cyclonic)
Spinning clockwise
Bending to the right
Spinning counter clockwise
Bending to the left

20. Hurricanes – Low Pressure
Hurricane is rising and already has moisture in it
Low pressure system at the surface – air rising so that means air is being sucked in at the base
Arrows get defected by Coriolis to the right
Set up a counter clockwise circulation in the northern hemisphere L

21. High Pressure H Air sinks and compresses and it gets warmer and dry
See clear air not clouds
Pushes air away
Rotate clockwise in northern hemisphere H

22. Atmospheric Circulation
Sinking air at 30° – deserts
Easterly winds – trade winds
Westerly winds at 30° and 60°

23. Atmospheric Circulation
3 convection cells in each hemisphere
Each cell: ~30° latitudinal width
Veritcal Motions
Rising Air: 0° and 60° Latitude
Sinking Air: 30° and 90° Latitude
Horizontal Motions
Zonal winds flow nearly along latitude lines
Zonal winds within each cell band
DUE to DEFLECTIONS BY CORIOLIS!

24. Sea Breeze
Land warms fastest during the day. Air during the day expands and rises
Ocean surface temperature changes slowly. Air cools and becomes more dense, sinks then begins to rise over the land.
Result – wind from sea towards land

25. Land Breeze
Land cools fastest at night. Low heat capacity. Air contracts and sinks
Ocean surface temperature changes slowly. Air is pushed away and up by cooler denser land air.
Result – wind from land towards sea

26. Marine Layer Cold waters, warm air: think cloud layer on ocean surface
Subtropics: H pressure, regional subsidence
Cloud layer flows onto land at night
Evaporates over land by day
LAND
OCEAN

27. Marine Layer Cold waters, warm air: think cloud layer on ocean surface
Subtropics: H pressure, regional subsidence
Cloud layer flows onto land at night
Evaporates over land by day
LAND
OCEAN

28. Marine Layer Cold waters, warm air: think cloud layer on ocean surface
Subtropics: H pressure, regional subsidence
Cloud layer flows onto land at night
Evaporates over land by day
LAND
OCEAN

29. Marine Layer Cold waters, warm air: think cloud layer on ocean surface
Subtropics: H pressure, regional subsidence
Cloud layer flows onto land at night
Evaporates over land by day
LAND
OCEAN

30. Marine Layer Cold waters, warm air: think cloud layer on ocean surface
Subtropics: H pressure, regional subsidence
Cloud layer flows onto land at night
Evaporates over land by day
LAND
OCEAN